If you saved before encountering a Legendary, you can soft reset to encounter it again until you find the right one you want, but saving after catching or fainting a Legendary, soft resetting can't bring the Legendary back. It won't mess up the soft reset, but it can ruin the chances of finding a shiny Legendary if you already saved after encountering it.
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I don't now but I believe that you can catch as many as you want when you encounter them.
It is completely random unless you cheat or keep on trying to get it as a shiny by continuously checking it out and lightly resetting the game until you get it as a shiny.
It is a rare pokemon, but it is not a legendary pokemon
If there are legendary Pokemon running around freely across Sinnoh, then it will show where they are. Note that whenever you go somewhere else (a new town, a new route, etc.) the Pokemon's position changes. If you happen to be lucky enough to be in the exact same area as the Pokemon, then go into any area where you encounter wild Pokemon and usually in the first couple of battles you will encounter the legendary Pokemon. Please note again that the second they can the Pokemon will try to flee from battle, so make sure that a speedy Pokemon who knows a move like Block is first in your party when you encounter the legendary Pokemon. You can also use the Marking Map to mark places that you want to remember. To do this, simply drag one of the shapes from the bottom of the screen and put it anywhere on the map. Good luck!
No, Zorua and Zoroark are not Legendaries. However they should be since the only way to get them is through a fateful encounter event from trading the Shiny Legendary dogs and Celebi over to Black or White.